1. Field of the Invention
The present invention relates to photo-assisted CVD (photo-assisted chemical vapor deposition) and, more particularly, to a photo-assisted CVD apparatus for forming semiconductor thin films having uniform quality at a high throughput and a photo-assisted CVD method.
2. Description of the Related Art
In recent years, a photo-assisted CVD method in which source gases such as silane and disilane are decomposed by using optical energy and chemically reacted to form a semiconductor thin film or the like has received a great deal of attention.
According to the photo-assisted CVD method, a film can be advantageously formed at a temperature lower than that of a conventional thermal CVD method in which a source gas is decomposed by using a thermal energy. In the photo-assisted CVD method, unlike a plasma CVD method, damage to a substrate, caused by charged particles, can be considerably suppressed because radicals are used as a species contributing to the formation of a film. The photo-assisted CVD method has high controllability, and a high-quality thin film can be formed.
FIG. 1 is a schematic view showing the arrangement of a conventional laminar flow type photo-assisted CVD apparatus using a mercury sensitization method.
In FIG. 1, a target substrate 106 is placed inside a reaction chamber 105. The target substrate 106 is heated by a heater 107. A lamp house 101 is arranged above the reaction chamber 105. A low-pressure mercury lamp 102 is meanderingly arranged in the lamp house 101 as shown in the plan view of the upper portion of FIG. 1, and the low-pressure mercury lamp 102 is connected to a power supply 113. An N.sub.2 gas is fed to the lamp house 101 as a purge gas to prevent ultraviolet attenuation caused by light absorption of atmospheric components (oxygen gas, water vapor, and the like).
A reflection plate 103 is arranged above the low-pressure mercury lamp 102. Light rays emitted from the low-pressure mercury lamp 102 are directly radiated on the target substrate 106 through a light-receiving window 104 or reflected from the reflection plate 103 to be radiated on the target substrate 106 through the light-receiving window 104.
A gas flow control plate 112 consisting of a material transparent to ultraviolet rays, e.g., quartz, is arranged below the low-pressure mercury lamp 102. A purge gas feed nozzle plate 111 transparent to ultraviolet rays and having a large number of through holes is arranged between the gas flow control plate 112 and the light-receiving window 104.
A mercury reservoir 109 for storing mercury whose temperature is kept constant, a source gas supply portion (not shown) for storing a source gas such as SiH.sub.4, a purge gas supply portion (not shown) containing an inert gas such as Ar serving as a purge gas, and a gas exhaust portion 108 constituted by a vacuum exhaust pump are arranged outside the reaction chamber 105. A mechanism for mixing a material absorbing ultraviolet rays with the purge gas may also be arranged.
A source gas A supplied from the source gas supply portion is fed into the reaction chamber 105 through the mercury reservoir 109 and a source gas feed nozzle 110a. That is, the source gas A and a mercury vapor flow in the reaction chamber 105.
A purge gas B supplied from the purge gas supply portion is sprayed on the target substrate 106 through a purge gas feed nozzle 110b, the purge gas feed nozzle plate 111, and the gas flow control plate 112. As a result, a gas mixture C of the source gas A and the mercury vapor flows parallelly to the target substrate 106 to form a laminar flow of the gas mixture C near the surface of the target substrate 106 and to form a laminar flow of the purge gas B at portions except for the portion near the surface of the target substrate 106.
In the photo-assisted CVD apparatus described above, in general, the size of an ultraviolet source (the low-pressure mercury lamp 102) serving as an optical energy source cannot easily be considerably larger than that of the target substrate 106 because the cost is increased. The ultraviolet source is larger than the target substrate 106 by one size. For this reason, the illuminance of ultraviolet rays is high at the central portion of the target substrate 106, and the illuminance of ultraviolet rays is low at the peripheral portion of the substrate. Therefore, a film forming rate at the central portion of the substrate is higher than that at the peripheral portion of the substrate, and a semiconductor thin film having uniform film quality and a uniform thickness cannot easily be obtained.
In the photo-assisted CVD method, in general, all reaction products are not always deposited on the substrate, and some of the reaction products are deposited on the inner wall of the reaction chamber. In this manner, when the film deposited on the inner wall of the reaction chamber is peeled, dust is produced in the reaction chamber, and a production yield is decreased. For this reason, the film deposited on the inner wall of the reaction chamber must be removed.
As a method of removing the film deposited on the inner wall of the reaction chamber, an overhaul cleaning method, a plasma discharge cleaning method, or the like is proposed. However, these methods have the following problems.
In the overhaul cleaning method, a reaction chamber is open to the atmosphere to remove a film deposited on the inner wall of the reaction chamber by chemical and mechanical methods. According to the method, the inner wall of the reaction chamber absorbs contaminants in the atmosphere. For this reason, a process of removing the contaminants, i.e., a vacuum exhaust process, is newly required, thereby decreasing the throughput in film formation.
On the other hand, in the plasma discharge cleaning apparatus, a deposit can be removed from the inner wall of the reaction chamber without exposing the inner wall to the atmosphere because the deposit is removed by plasma etching. In order to remove a deposit by the plasma discharge cleaning method in a photo-assisted CVD apparatus, plasma discharge electrodes (discharge electrodes) must be arranged in the reaction chamber. As shown in FIGS. 2A and 2B, two discharge electrodes whose longitudinal direction is parallel to a source gas flow are arranged on both the sides of the substrate. The discharge electrodes are arranged not to interfere with the flow of the source gas and irradiation of light.
The etching rate of the film deposited between the discharge electrodes in the reaction chamber is highest at portions near the discharge electrodes, decreased at a position far from the discharge electrodes, and lowest at a portion near the central portion between the discharge electrodes. For this reason, the time required for cleaning the inside of the reaction chamber is considerably prolonged, and a throughput is decreased.
As described above, in the conventional photo-assisted CVD apparatus, since the illuminance distribution of ultraviolet rays on the substrate is not uniformed, a semiconductor thin film having a uniform film thickness and uniform film quality or the like cannot easily be formed.
A film deposited on the inner wall of the reaction chamber must be removed before the film formation to prevent a decrease in yield. Although the deposit can be removed by the plasma discharge cleaning method without exposing the inner wall to the atmosphere, the time required for cleaning the reaction chamber is prolonged because the arrangement of the discharge electrodes is restricted. For this reason, a throughput is disadvantageously decreased.